Throughout their lives, plants are routinely subjected to a variety of stresses which act to impede or alter growth and development processes. Because it negatively impacts plant growth and development processes, stress to agricultural plants has a negative economic impact in the form of reduced yields, increased expenditures to ameliorate the effects of stress, or both. Given the world's increasing human population and the diminishing land area available for agriculture, improving agricultural productivity is of paramount importance. Thus, there is a need for crop plants that are better able to tolerate stresses and maintain productivity under unfavorable conditions.
While traditional plant breeding approaches will continue to be important for improving agricultural plants, the new strategies that are likely to have the most significant impact on crop improvement will involve genetic engineering. A thorough understanding of the molecular and cellular mechanisms used by plants to avoid or tolerate stresses may help in the development of new strategies to improve the stress tolerance of agricultural plants.
Stresses to plants may be caused by both biotic and abiotic agents. For example, biotic causes of stress include infection with a pathogen, insect feeding, parasitism by another plant such as mistletoe, and grazing by animals. Abiotic stresses include, for example, excessive or insufficient available water, excessive or insufficient light intensity, temperature extremes, synthetic chemicals such as herbicides, and excessive wind. Yet plants survive and often flourish, even under unfavorable conditions, using a variety of internal and external mechanisms for avoiding or tolerating stress. Plants' physiological responses to stress reflect changes in gene expression.
While manipulation of stress-induced genes may play an important role in improving plant tolerance to stresses, it has been shown that constitutive expression of stress-inducible genes has a severe negative impact on plant growth and development when the stress is not present. (Kasuga et al., Nature Biotechnology 17:287–291) Therefore, there is a need in the art for methods employing regulatory elements responsive to stress, to provide a means to control and direct expression of genes involved in stress tolerance.